Security or alarm systems are installed in premises to detect hazardous or potentially hazardous conditions. A security system generally includes a plurality of detectors/sensors organized into zones, one or more keypads and a control panel which contains the system electronics and may include a communication interface for remote monitoring and two-way communication over telephone or wireless communication paths. Each of the detectors communicates with the control panel to provide notification of an alarm condition. Examples of possible alarm conditions include unauthorized entry or the unexpected presence of a person who may be an intruder, fire, smoke, toxic gas, high/low temperature conditions (e.g., freezing), flooding, power failure, etc. In other words, an alarm condition may represent any detectable condition that might lead to personal hazard or property damage. Audible and/or visible alarm devices such as sirens, lights, etc., may also be utilized to notify occupants of the existence of an alarm condition. The control panel may be located in a utility room, basement, etc., and may communicate with the detectors and notification devices by wired or wireless signal paths. A keypad, which may also communicate with the control panel via a wired or wireless connection, is used to arm/disarm the system as well as providing a means to display various system messages via a status display screen.
Monitoring and detecting the present zone status for a security alarm system is of great importance to ensure the proper functioning of the system. Existing zone signaling circuits and methods mostly rely on a DC voltage drop reading across a resistor to read end of line states. Such methods and circuits, however, are not as immune to common mode noise (e.g., zone loop is floating) or voltage additive noise (e.g., zone loop is a current loop). In addition, the DC voltage drop across a resistor methodology may not be as tamper resistant nor provide the level of zone isolation as the embodiments described herein. Other prior art systems may use complicated analog circuitry to achieve acceptable zone isolation. Complex circuitry increases the cost to the overall system, however, and for multiple inputs the solution may become non-marketable.